Variable speed wind turbine having a passive grid side rectifier with scalar power control and dependent pitch control

ABSTRACT

A variable speed wind turbine having a passive grid side rectifier using scalar power control and dependent pitch control is disclosed. The variable speed turbine may include an electrical generator to provide power for a power grid and a power conversion system coupled to the electrical generator. The power conversion system may include at least one passive grid side rectifier. The power conversion system may provide power to the electrical generator using the passive grid side rectifier. The variable speed wind turbine may also use scaler power control to provide more precise control of electrical quantities on the power grid. The variable speed wind turbine may further use dependent pitch control to improve responsiveness of the wind turbine.

RELATED APPLICATION

This application is a divisional of application Ser. No. 10/074,904,entitled “VARIABLE SPEED WIND TURBINE HAVING PASSIVE GRID SIDE RECTIFIERWITH SCALAR POWER CONTROL AND DEPENDENT PITCH CONTROL, filed on Feb. 11,2002, which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to variable speed wind turbines,and, more particularly, to a variable-speed wind turbine having apassive grid side rectifier with scalar power control and dependentpitch control.

BACKGROUND OF THE INVENTION

A wind turbine is an energy converting device. It converts kinetic windenergy into electrical energy for utility power grids. This type ofenergy conversion typically involves using wind energy to turn windblades for rotating a rotor of an electrical generator. Specifically,wind applied to the wind blades creates a force on the rotor, causingthe rotor to spin and convert the mechanical wind energy into electricalenergy. Hence, the electrical power for such a generator is a functionof the wind's power. Because wind speed fluctuates, the force applied tothe rotor can vary. Power grids, however, require electrical power at aconstant frequency, such as 60 Hz or 50 Hz. Thus, a wind turbine mustprovide electrical power at a constant frequency that is synchronized tothe power grids.

One type of wind turbine that provides constant frequency electricalpower is a fixed-speed wind turbine. This type of turbine requires agenerator shaft that rotates at a constant speed. One disadvantage of agenerator shaft that rotates at a constant speed is that it does notharness all of the wind's power at high speeds and must be disabled atlow wind speeds. That is, a generator limits its energy conversionefficiency by rotating at a constant speed. Therefore, to obtain optimalenergy conversion, the rotating generator speed should be proportionalto the wind speed.

One type of wind turbine that keeps the rotating generator speedproportional to the wind speed is a variable speed wind turbine.Specifically, this type of turbine allows a generator to rotate atcontinuously variable speeds (as opposed to a few preselected speeds) toaccommodate for fluctuating wind speeds. By varying rotating generatorspeed, energy conversion can be optimized over a broader range of windspeeds. Prior variable speed wind turbines, however, require complicatedand expensive circuitry to perform power conversion and to control theturbine.

One prior variable speed wind turbine is described in U.S. Pat. No.5,083,039, which describes a full power converter having a generatorside active rectifier coupled to a grid side active inverter via adirect current (DC) link. In this configuration, the active rectifierconverts variable frequency AC signals from the generator into a DCvoltage, which is placed on the DC link. The active inverter convertsthe DC voltage on the DC link into fixed frequency AC power for a powergrid. A disadvantage of such a configuration is that it requirescomplicated and expensive circuitry utilizing active switches (e.g.,insulated-gate bipolar transistors IGBTs) for the active rectifier andinverter. These types of active switches typically have higher powerloss during power conversion and cause unwanted high frequency harmonicson the power grid. Furthermore, both the active rectifier and invertermust be controlled. Moreover, active components are less reliable thanpassive components.

Another prior variable speed wind turbine is described in U.S. Pat. No.6,137,187, which includes a doubly-fed induction generator and aback-to-back power converter. The power converter includes a generatorside converter coupled to a grid side converter via a DC link. Both thegenerator and grid side converters include active switches. The turbinedescribed in the '187 patent is a partial conversion system because onlya portion of the generator's rated power ever passes through theback-to-back converter. Moreover, unlike the power converter of the fullconversion system, power flows through the converter in oppositedirections. That is, power can flow to the rotor windings from the powergrid in order to excite the generator or power can flow from the rotorwindings to supplement the constant frequency AC power from the statorwith constant frequency AC power from the rotor.

To supply power from the power grid to the rotor windings through theback-to-back converter, the grid side converter acts as a rectifier andconverts constant frequency AC signals into a DC voltage, which isplaced on the DC link. The generator side converter acts as an inverterto convert the DC voltage on the DC link into variable frequency ACsignals for the generator, so as to maintain constant frequency power onthe stator. To supply power from the rotor windings to the grid throughthe back-to-back converter, the generator side converter acts as arectifier and converts variable frequency AC signals into a DC voltage,which is placed on the DC link. The grid side converter then acts as aninverter to convert the DC voltage on the DC link into fixed frequencypower for the grid. A disadvantage of this type of back-to-backconverter is that it requires complicated and expensive circuitryutilizing active switches for both converters. As stated previously,using active switches can typically cause unwanted power loss duringpower conversion and unwanted high frequency harmonics on the powergrid. Furthermore, like the prior full power converter, both convertersmust be controlled, and active components are less reliable than passivecomponents.

One type of control of the generator side converter involvestransforming AC signals representing three phase generator electricalquantities into parameters with a coordinate transformation so that thegenerator can be controlled using DC values (which is known asPark-transformation). This type of control is a form of “field orientedcontrol” (FOC). A disadvantage of using FOC-type control is that usefulinformation regarding the AC signals may-be lost in the transformationprocess. Specifically, FOC assumes that the AC signals of the threephases are symmetrical (that is, that they only differ in phase). Incertain instances, the AC signals are asymmetrical and useful ACinformation may be lost during the transformation from AC signals intoDC values.

Furthermore, because FOC loses information when transforming to DCvalues, FOC is unable to be used in a system that independently controlsthe electrical quantities (e.g., voltage, current) of each phase of thepower grid. Theoretically, this should not pose a problem because theelectrical quantities for each phase of an ideal power grid should notvary. In actuality, however, the electrical quantities on each phase ofthe power grid may vary, causing uneven thermal stress to develop on thegenerator and non-optimal power generation. Accordingly, it would bedesirable to independently control these electrical quantities for eachof the three phases of the power grid.

Another aspect of a wind turbine is a pitch controller. Typicalgenerators ramp up to a preselected constant speed of operation, knownas “rated speed.” When the generator is operating at, or just beforereaching, rated speed, the turbine controls the angle at which theturbine's blades face the wind, known as the “pitch angle” of theblades. By controlling the pitch angle, the turbine can maintain thegenerator at a rated speed. Pitch controllers, however, typicallyoperate at a low frequency as compared to power conversion controllers.Thus, pitch controllers are slow to react to rapid changes in speed,which are typically caused by wind gusts.

SUMMARY OF THE INVENTION

One aspect of the present invention discloses a variable speed windturbine. For example, the variable speed turbine may include anelectrical generator to provide power for a power grid and a powerconversion system coupled to the electrical generator. The powerconversion system may include at least one passive grid side rectifierto power to the electrical generator. Another aspect of the presentinvention discloses a variable speed wind turbine that may use scalarpower control to provide more precise control of electrical quantitieson the power grid. Still another aspect of the present inventiondiscloses a variable speed wind turbine that may use dependent pitchcontrol to improve responsiveness of the wind turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in, and constitute apart of, this specification illustrate implementations of the inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings,

FIG. 1 illustrates one implementation of a circuit diagram for avariable speed wind turbine having a passive grid side rectifierconfiguration;

FIG. 2 illustrates a flow diagram of a method to control the powerdissipating element of FIG. 1 at below and above synchronous speed;

FIG. 3 illustrates a block diagram of one implementation of a scalarpower control and dependent pitch control processing configuration for avariable speed wind turbine;

FIG. 4 illustrates a processing flow diagram of one implementation ofscalar power control, which can be used by the power controller of FIG.3;

FIG. 5 illustrates a flow diagram of a method for performing scalarpower control using controllable oscillating signals;

FIG. 6 illustrates an internal block diagram of one implementation forthe main controller of FIG. 3;

FIG. 7 illustrates aspects of one implementation for the partial loadcontroller of FIG. 6,

FIG. 8 illustrates aspects of one implementation for the full loadcontroller of FIG. 6; and

FIG. 9 illustrates a block diagram of one implementation for the pitchcontroller of FIG. 3.

DETAILED DESCRIPTION

Reference will now be made in detail to implementations of theinvention, examples of which ire illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

The variable speed wind turbine described herein provides a simplifiedpower converter using a passive grid side rectifier, which avoids usingactive switches. For example, the passive rectifier could be comprisedof diodes. As such, the passive grid side rectifier does not requireprocessor control and provides for a more reliable power converter. Inparticular, passive components are more reliable than active components.Furthermore, because active switches can cause power loss during powerconversion, the passive grid side rectifier can improve power conversionefficiency for the wind turbine. In addition, using a passive grid siderectifier does not produce high frequency harmonics and provides lessexpensive and complicated circuitry for a power converter in the windturbine.

The wind turbine also provides instantaneous control of rotor currentsof a generator to control the instantaneous power provided to a powergrid (“scalar power control”). Scalar power control can be responsive tothe actual electrical characteristics for each phase of a power grid.

The wind turbine further uses dependent pitch control that is dependenton the power controller (“dependent pitch control”). In particular, oneimplementation discloses a low-speed pitch controller that receivessignals or information from a high-speed power controller, therebyimproving the responsiveness of the pitch controller.

As described in further detail below, the variable speed wind turbinemay be implemented with a doubly-fed wound rotor induction generator toproduce electrical power. The generator may operate at below synchronousspeed and above synchronous speed.

Synchronous speed is the speed at which a rotor (mechanical speed) isrotating at the same speed as the magnetic fields in a stator. In thecontext of the wind turbine described below, synchronous speed can be1800 rpm. Typically, the stator frequency is fixed to the power gridfrequency. In the United States, the nominal power grid frequency is 60Hz, meaning that the stator frequency is 3600 rpm. For a generatorhaving four poles (or two pole pairs), the generator's synchronous speedwould be 3600 rpm/2 or 1800 rpm. In the following implementations,operation at below synchronous speed refers to a generator speed orrotor speed that is below 1800 rpm. Operation at above synchronous speedrefers to a rotor speed that is above 1800 rpm. The precise value forsynchronous speed in the context of this description depends on factorssuch as generator design (e.g., number of pole pairs) and utility gridfrequency (e.g., 50 Hz in Europe). The wind turbine described below canbe designed to operate at any desired synchronous speed.

In the implementations described herein, by controlling the activeelements of an electrical generator's rotor side converter or inverterand by controlling the pitch of the turbine blades, a desired amount ofconstant frequency power may be supplied from the generator's statorwindings. At rotor speeds below synchronous speed, excitation power canbe supplied to the generator's rotor from a power grid using the passivegrid side rectifier. At rotor speeds above synchronous speed, power flowcan be reversed due to excess power from the electrical generator'srotor, which requires that the excess power be dissipated in the powerconverter.

Passive Grid Side Rectifier Configuration

FIG. 1 illustrates a circuit diagram of one implementation for thevariable speed wind turbine 100 having a passive grid side rectifierconfiguration consistent with the invention. Wind turbine 100 includesan electrical generator 110 having a stator 113 and a rotor 12 connectedto a generator rotor shaft 111. Although not shown, generator rotorshaft 111 is connected to wind blades for wind turbine 100. Animplementation of this connection may be through a gear box (as shown inFIG. 3 at 302). In one embodiment, generator 110 is implemented as adoubly-fed wound rotor induction generator such that rotor 112 andstator 113 both include two-pole, 3 phase windings to generate electricpower from rotation of rotor shaft 111. Generator 110 supplies fixedfrequency AC signals (electrical power) to the power grid (“grid”) fromstator 113. Rotor 112 may receive slip and excitation power for theoperation of generator 10 from the power conversion system (“powerconverter”) 150 through a passive grid side rectifier 154. Rotor 112 mayalso direct excess generated power to converter 150, which can dissipatethe excess generated power.

Generator 110 is coupled to the power converter 150 via inductors 140.Inductors 140 act as a filter to prevent large voltage changes on thewindings within generator 110. Power converter 150 is coupled to powertransformer 180. Power transformer 180 may be, for example, a 690V/480Vpower transformer with an integrated choke or separated choke“inductor.” In particular, power transformer 180 supplies 690V to thegrid and 480V to power converter 150. Power transformer 180 is coupledto a grid charge circuit including switches 145 and resistors 146 tocharge power converter 150, without significant inrush current, by powertransformer 180. This circuit is also coupled to an electromagneticcompatibility (EMC) filter 140, which filters harmonic distortion causedby power converter 150. An over-voltage protection (OVP) circuit 160 isalso coupled to generator 110. OVP circuit 160 operates to protect powerconverter 150 from damage in over-voltage conditions.

Generator 110 supplies power to the grid via stator 113. Stator 113connects to the grid via a delta (“Δ”) connector 131A and main connecter105 or via a Y connector 131B and main connector 105. The Δ connector131A and main connector 105 can configure windings in stator 113 so thatthey are in a Δ connection. The Y connector 131B and main connector 105can configure windings in stator 113 so they are in a Y connection. Inone implementation, the same stator windings are used for the Δ and Yconnections. In this manner, a Y-connection reduces iron losses instator 113 and permits a wider speed range for low wind speeds. Thus,generator 110 can selectively provide electrical power to the grid fromstator 113 via A connector 131A and main connecter 105 or Y connector131B and main connecter 105. Furthermore, this allows wind turbine 100to reduce power loss by selectively connecting generator 110 to the gridusing the delta, connector 131A and main connecter 105 or the Yconnector 131B and main connecter 105. The grid operates as a 3-phase690V utility power grid at a fixed frequency such as 60 Hz. The grid mayalso operate at other voltages or fixed frequencies, such as 50 Hz, orwith a different number of phases.

Power Converter

Variable speed wind turbine 100 includes a converter processor 170coupled to power converter 150 to control components within turbine 100,including regulating the turbine's output power flow and controllingcomponents, such as power converter 150. In one embodiment, converterprocessor 170 controls active components in power converter 150 so as tocontrol total electrical quantities supplied to the grid. Suchelectrical quantities may include the total current and power suppliedto the grid. The operation of controlling power converter 150 byconverter processor 170 will be described in more detail below.

Power converter 150 includes an active generator side inverter (“activeinverter 151”), DC link 152, power dissipating element 153, and passivegrid side rectifier 154 (“passive rectifier 154”). For purposes ofillustration, power converter 150 is shown with single elements;however, any number of elements may be implemented in power converter150. For example, power converter 150 may include any number of passiverectifiers in parallel with passive rectifier 154. Such a configurationwould be particularly useful in situations where a wind turbine providesa low power mode and at least one higher power mode, where one or moreof the parallel rectifiers would be enabled in the higher powergenerator mode(s). Multiple power dissipating elements 153 may also beprovided.

Normal operation for wind turbine 100 is at below synchronous speed suchthat power flow is directed from power converter 150 to generator 110.Consequently, in most instances, the components of active inverter 151operate as an inverter to convert DC voltage on DC link 152 intovariable frequency AC signals for generator 110. Thus, in the followingimplementations, active inverter 151 is referred to as an “inverter.” Incertain instances, however, wind turbine 100 may operate at abovesynchronous speed such that power flow is reversed (i.e., excess poweris being generated from generator 110) and the components of activeinverter 151 may be used as a rectifier. That is, when power flow isreversed, active inverter 151 operates to convert excess power beinggenerated from generator 110 into a DC voltage for power converter 150.This excess power can be dissipated or discharged by dissipating element153, which will be explained in further detail below.

Active inverter 151 includes active components or switches in athree-phase bridge configuration. In one embodiment, the active switchesare IGBTs. These active switches may be other types of switches, suchas, for example, bipolar junction transistors or field effecttransistors.

In one embodiment, pulse width modulated (PWM) current regulationtechniques are used to selectively control the active switches in activeinverter 151 under a scalar control algorithm (“scalar controlalgorithm”), as described below. The scalar control algorithm allows forindividual and/or independent PWM control for each phase of rotor 112based on measured electrical quantities for each phase of the grid. Thescalar control algorithm can control, individually and/or independently,electrical quantities for each phase of the grid. While other methods ofcontrol could also be employed, such as torque control using fieldoriented control, such as that described in the '187 patent, FOC-typecontrol is implemented in a different way, performs different functions,and achieves poorer results than the power control method describedherein.

The operation of active inverter 151 at below and above synchronousspeed will now be explained. At below synchronous speed, active inverter151 acts as an inverter, converting DC voltage on DC link 152 intovariable frequency AC signals that are supplied to generator 110. Atabove synchronous speed, active inverter 151 acts as a rectifier,converting variable frequency AC signals from generator 110 to a DCvoltage, which is placed on DC link 152. As will be described in furtherdetail below, when the DC voltage on DC link 152 exceeds a threshold,power dissipating element 153 will lower the voltage on DC link 152 byburning off excess power that is generated from generator 110.

DC link 152 includes a series of capacitor elements. One or more sets ofresistors can be added in some implementations to discharge thecapacitor elements and improve symmetry. In particular, the voltage dropacross each portion of the link should be substantially the same (orsubstantially symmetrical). DC link 152, however, may be implementedwith other types of voltage storage circuit configurations.

The operation of DC link 152 at below and above synchronous speed willnow be explained. At below synchronous speed, DC link 152 stores aconstant DC voltage, which can be mathematically calculated from thevoltage from power transformer 180 that is placed on the passiverectifier 154. In the case of the voltage from power transformer 180being 480V, the DC link voltage is 480V×√{square root over (2)}. Atabove synchronous speed, the voltage on DC link 152 may increase becausepower generated from rotor 112 charges DC link 152.

Power dissipating element 153 includes a pair of active switches(switches of this type are typically sold as pairs) having a commonconnection to a burn-off resistor and inductor connected in series. Theburn off resistor can be used to discharge excess voltage on DC link152, thereby dissipating excess power being generated from generator110. The inductor can be used in some implementations to reduce currentripple in power dissipating element 153 to protect it from damage. Theupper switch is either controlled or permanently biased into a highimpedance or “off” condition. Thus, in an alternate embodiment, only thelower switch may be provided. Additionally, the order of the circuitcomponents, e.g., the controlled switch, the resistor, and the inductorin the embodiment of FIG. 1, can be altered. In sum, power dissipatingelement can employ any structure to dissipate excess power on DC link152.

The operation of power dissipating element 153 at below and abovesynchronous speed will now be explained. At below synchronous speed, thelower switch is turned off such that power dissipating element 153 actsas an open circuit, which allows DC voltage from passive rectifier 154to be stored in DC link 152. At above synchronous speed, the lowerswitch can be selectively turned on to allow excess voltage on DC link152 to be discharged in the burn off resistor. In this process, excesspower from rotor 112 is being dissipated at above synchronous speed.

Passive rectifier 154 can include six power rectifier diodes connectedin a three phase bridge configuration. The operation of passiverectifier 154 at below and above synchronous speed will now beexplained. At below synchronous speed (a condition where the relativegrid voltage is higher than the DC link voltage), passive rectifier 154operates to convert fixed frequency AC signals from the power grid intoa DC voltage. The DC voltage from passive rectifier 154 is placed on DClink 152 to maintain the DC link voltage at a predetermined voltage.

In one embodiment, if the lower active switch in power dissipatingelement 153 is turned off, power dissipating element 153 acts as an opencircuit and the DC voltage from passive rectifier 154 passes directly toDC link 152. At above synchronous speed when power is being generatedfrom generator 110, the DC link voltage will exceed the grid voltage.The diodes of passive rectifier 154 act to prevent conversion of the DClink voltage into a current. Accordingly, passive rectifier 154 does notoperate to supply power to the grid. Moreover, the diodes comprisingpassive rectifier 154 and the power dissipating element 153 are designedto prevent breakdown of the diodes at times when the high DC linkvoltage is discharged by power dissipating element 153.

Converter Processor

Converter processor 170 can be used as the power controller and internalcontrol and supervision of power converter 150 for wind turbine 100. Inone embodiment, converter processor 170 controls the active componentsor switches in active inverter 151 using scalar power control with ascalar control algorithm as described in FIGS. 4 and 5. Converterprocessor 170 can also control power dissipating element 153 using themethod described in FIG. 2.

To control these active switches using the scalar power control with thescalar control algorithm, converter processor 170 uses input signalssuch as generator speed f_(gen), grid voltage U_(grid), grid currentI_(grid), and measured rotor current values I_(rotor), for, for eachphase of rotor 112 (IR1, IR2, IR3). Converter processor 170 also uses agrid frequency signal indicating the operating frequency of the grid,which can be calculated from the U_(grid) signal. These input signalsand the grid frequency signal allow converter processor 170 to controlpower to the grid without performing a coordinate transformation of ACsignals. This allows for precise control of electrical quantities foreach phase of the grid because information regarding each phase of thegrid is maintained (as opposed to being lost in a transformationprocess).

Generator speed can be measured or derived, in a sensor-less system,from measured electrical quantities. Generator speed is used to control,among other things, the frequency of the PWM control of inverter 151.

I_(grid) and U_(grid) indicate current and voltage measurements,respectively, on the grid. These measurements can represent current andvoltage measurements for each phase of the grid. I_(grid) and U_(grid)are also used by converter processor 170 to calculate active andreactive power and reference waveforms to control individually andindependently electrical quantities on the grid. More specifically,these signals can be used to control current and active and reactivepower for each phase of the grid as will be explained in further detailbelow.

To control power dissipating element 153 using the method of FIG. 2,converter processor 170 uses a signal line connected to powerdissipating element 153. Also, converter processor 170 receives a sensedvoltage level on DC link 152 using the “DC link” signal line as shown inFIG. 1. At a normal state (below synchronous speed), the voltage levelon DC link 152 is at an acceptable threshold. At an abnormal state(above synchronous speed) caused by, e.g., a sudden wind gust, thevoltage level on DC link 152 may be above the acceptable threshold. Thisis caused by generator 110 creating excess power because of the windgust. In this situation, converter processor 170 can send a controlsignal over the connecting signal line to the lower active switch inpower dissipating element 153 such that the excess generated power isburned off or discharged in power dissipating element 153. In analternative embodiment, converter processor can control powerdissipating element 153 with time-varying signals, such as by usingpulse width modulation (PWM) signals so as to avoid overstressing powerdissipating element 153. One example of this PWM control would becontrolling the power dissipating element 153 like a brake chopper. Forinstance, the active switches in power dissipating element 153 can beselectively “turned on” or “turned off” with a selected duty cycle. Theduty cycle can be adjusted based on the DC link voltage.

The above description provides exemplary implementations of converterprocessor 170. Converter processor 170 may, alternatively oradditionally, include, e.g., separate drive circuits and controllers todrive and control the active switches in converter 151 and powerdissipating element 153. Converter processor 170 may also receive othertypes of input signals such as U_(sync). U_(sync) can represent avoltage measurement created by the magnetic buildup on stator 113 ofgenerator 110. U_(sync) can be used at start up of wind turbine 100 inthat it provides an indication of when generator 110 is to be connectedto the grid. For example, if U_(sync) is synchronized with the U_(grid)signal, generator 10 can be connected to the grid in this instance.

Power Dissipating Element Control

FIG. 2 illustrates a flow diagram of a method to control powerdissipating element 153 by converter processor 170 in FIG. 1 at belowand above synchronous speed. Initially, the process begins at stage 202.At this stage, converter processor 170 senses a voltage on DC link 152and determines if the voltage is above a threshold. For example,converter processor 170 may receive sensed DC voltage levels for DC link152 using a “DC link” input signal as shown in FIG. 1. In oneembodiment, the threshold is set above the normal voltage level or valueon DC link 152 at below synchronous speed, which may equal √{square rootover (2)} times the voltage for power converter 150. For example, thethreshold may be set above 480V×√{square root over (2)}. The thresholdvoltage may also be set at other voltage levels such as above690V×√{square root over (2)} if power converter 150 operates at 690V.The threshold voltage is preferably below a level based on the DC link152 voltage ratings to avoid damaging DC link 152. If the voltage is notabove the threshold, converter processor 170 at stage 204 maintains theactive switches in power dissipating element 153 in an off position.Because the voltage on DC link 152 is not greater than the threshold, itcan be determined that generator 110 is operating at below synchronousspeed. Thus, no measurement of generator speed is necessary to make adetermination of whether generator 110 is operating at below synchronousspeed.

On the other hand, if converter processor 170 determines the voltage onDC link 152 is above the threshold, converter processor 170 at stage 206controls power dissipating element to turn on such that the excessvoltage from DC link 152 (or power from the rotor of generator 110 atabove synchronous speed) is discharged. In one embodiment, after thisstage, converter processor 170 can turn off the power dissipatingelement 153 if it senses that the voltage on DC link 152 is at a normaloperating level such as, for example, 480V×√{square root over (2)}. Inan alternative embodiment, converter processor 170 may turn off theswitches at a different voltage level that is acceptable for operatingturbine 100. For example, the power dissipating element 153 can bedisabled at a voltage lower than the voltage used to enable powerdissipating element 153, providing hysteresis. This threshold can alsobe adjustable or configurable based on the operating environment ofturbine 100. After the power dissipating element is turned off, theprocess may continue at stage 202 again to determine if the voltage onDC link 152 is above a threshold, or alternatively, the process may end.

Scalar Power Control and Dependent Pitch Control ProcessingConfiguration

FIG. 3 illustrates one example of a block diagram of a scalar powercontrol and dependent pitch control processing configuration forvariable speed wind turbine 100 consistent with the invention. Referringto FIG. 3, the basic components for wind turbine 100 include a generator110 having a rotor 112 and a stator 113. Stator 113 connects andprovides electrical power created by generator 110 to the grid. Rotor112 converts mechanical energy, which is provided by wind blades 301,into electrical energy for generator 110. Although two wind blades areshown, three wind blades, or any number of wind blades, may be used forwind turbine 100. Wind blades 301 connect to generator 110 via a mainshaft 303, gear box 302, and generator rotor shaft 111. Gear box 302connects main shaft 303 to generator rotor shaft 111 and increases therotational speed for generator rotor shaft 111.

The control processing configuration (“control system”) for wind turbine100 can be implemented in hardware as a multi-processor system. Forexample, although not shown, the control system may include a groundprocessor hardware unit, which is located at the bottom of the tower ofa turbine, a top processor hardware unit, which is located in thenacelle of the turbine (not shown), a hub processor turbine unit, whichis located in the hub of the turbine and rotates with turbine's blades,and a converter processor hardware unit, which is located in thenacelle. Each of these hardware units may include one or more processorchips and may be connected to each other by a suitably fast andefficient network to enable data transfer between the units, such as anAttached Resource Computer Network (ARCnet). Other interfacing protocolscould alternatively be used, such as Controller Area Network (CAN),Ethernet, FDDI, Token Ring and local area network (LAN) protocols.

Functionally, the control system may include a number of controllers forcontrolling components within wind turbine 100 as shown in FIG. 3.Parameters such as communication speed, sample time requirements, andprocessing capacity determine where portions of the functional blocksare physically computed (that is, which operations are performed inwhich hardware unit). For example, in one implementation, the functionsof the power controller are physically computed within the converterprocessor. Operations for a single functional block may also beperformed in a number of hardware units.

The control system includes a main controller 310 coupled to a powercontroller 312 and pitch controller 316. Main controller 310 can be usedto control the overall functions for wind turbine 100. Pitch controller316 is dependent on power controller 312 through a power error feedforward 314. Pitch controller 316 controls the pitch angle for windblades 301. In one embodiment, power controller 312 can control gridcurrents for each respective phase of the grid and, thereby, controlactive and, reactive power on the grid. Power controller 312 alsocontrols power converter 150 to provide power to generator 110 and todischarge or burn off excess power from generator 110.

Main controller 310 generates and provides a main pitch reference signalto pitch controller 316 and a power reference signal (PMG_(ref)) topower controller 312. The manner in which main pitch reference signaland PMG_(ref) signal are generated will be discussed in further detailbelow. To generate the main pitch reference and PMG_(ref) signals, maincontroller 310 processes received measurements as described in moredetail in FIGS. 6 through 8. Main controller 310 may also receivecommands from a user or other internal or external processing units.Main controller 310 may also receive other types of input signals suchas, for example, temperature measurement signals indicating-temperaturereadings of components or status signals on whether switches orconnections or “on” or “off” in wind turbine 100. Such input signals maybe used to control the overall operation and supervision of wind turbine100.

Power controller 312 receives PMG_(ref) signal from main controller 310to determine a power error signal. The power error signal may includeinformation related to a calculated error for active and reactive powerbased on current and voltage levels for each phase of the grid. Forexample, power controller 312 may calculate the power error signal asthe magnitude of the target real power minus the magnitude of themeasured real power. Power controller 312 also receives a generatorspeed signal from generator 110, which may be used to control componentsin power converter 150. Power controller 312 may also receive the sameinputs signals for converter processor 170 as shown in FIG. 1. Thus,power controller 312 may receive the U_(grid), I_(grid), generatorspeed, and current measurement t signals. Power controller 312 usesthese signals to control grid currents for each phase of the grid and,thereby, active and reactive power.

Power error feed forward 314 receives the power error signal from powercontroller 312 and processes this signal to determine the secondarypitch reference signal. Power error feed forward 314 allows fordependency between pitch controller 315 and power controller 312. Thefunctions of power error forward feed 314 can be performed in any of thehardware units within wind turbine 100, e.g., the top processor hardwareunit. Power error feed forward 314 allows for quick reaction time forpitch controller 316 to respond to errors detected by power controller312. That is, power error feed forward 314 ensures a quick and reliablereaction by pitch controller 316 to control the pitch for wind blades301 so as to maintain stability for wind turbine 100.

For example, power error feed forward 314 may receive the power errorsignal (i.e., the magnitude of the target real power minus the magnitudeof the measured real power) from power controller 312. Based on anonlinear table, power error feed forward 314 generates the secondarypitch reference signal for pitch controller 316. In other words, if thepower error signal is considerably high, e.g., in one embodiment higherthan 20% of nominal power, this would indicate that the power fromgenerator 110 is lower than expected, which means a risk of strongacceleration that may lead to an overspeed condition for generator 110.Power error feed forward 314 would thus set the secondary pitchreference signal to a nonzero value based on the power error from powercontroller 312 and the actual pitch angle from wind blades 301 tocompensate for the error. If the power error is within tolerances, thesecondary pitch reference is set to zero.

Although described in a multi-processor system, a single processor canbe used to implement the functions performed by pitch controller 316,power controller 312, power error feed forward 314, and main controller310. In particular, the functions for these controllers can be embodiedin software, which can be executed by a processor to perform theirrespective functions.

Scalar Power Control

Wind turbine 100 uses scalar power control to control total power andtotal current levels for each phase of the grid. This avoids usingcomplicated and expensive FOC processing. One purpose of scalar powercontrol is to provide a constant power output from the generator for agiven wind speed. Furthermore, scalar power control, as described below,provides more precise control of electrical quantities for each of thethree phases of the grid so as to provide optimum operation for thegrid. To implement scalar power control, wind turbine 100 uses a powercontroller 312 operating a scalar control algorithm described in FIG. 5.The following scalar power control techniques can be implemented with atime-based system. Specifically, measurements taken in real time orinstantaneously can be used to provide scalar power control.

Power Controller

FIG. 4 illustrates one example of a processing flow diagram for thepower controller 312 of converter processor 170. At processing stage402, U_(grid) and I_(grid) signals are received. U_(grid) providesvoltage measurement information for each of the three phases of the gridrepresented as u_(L1)-u_(L3). I_(grid) provides current measurementinformation for each of the three phases of the grid represented asi_(L1)-i_(L3). Each voltage and current measurement for each phase isused to calculate active and reactive power, as detailed in FIG. 5. Thecalculated active power, which is represented as PMG, and the reactivepower, which is represented as QMG, are directed to processing stages403A and 403B, respectively.

At processing stages 403A and 403B, a PMG_(ref) signal and a QMG signalare received. These signals represent ideal active power values for aparticular wind speed and derived reactive power. At these stages, PMGand QMG values are compared with PMG_(ref) and QMG_(ref) values. Theinformation related to the comparison is sent to power controlprocessing stage 405. At processing stage 405, a calculated gridfrequency and generator speed information are received. This informationalong with information from processing stages 403A and 403B are used tocalculate current reference values IR1_(ref)-IR3_(ref). These values aredirected to processing stages 408A-408C, respectively. At processingstages 408A-408C, measured rotor currents IR1-IR3 are received fromrotor 112. Processing stages 408A-408C compares the measured currentvalues IR1-IR3 with their respective current reference valuesIR1_(ref)-IR3_(ref). The comparison information is sent to currentcontrol processing stages 410A-410C.

The current control processing stages 410A-410C determine PWM controlsignals UR1 _(ref)-UR3 _(ref), which are sent to a PWM processing module420. PWM processing module uses these signals to control the activeswitches in active inverter 151, which then outputs new rotor currentsIR1-IR3. Because the U_(grid) and I_(grid) values for each phase on thegrid are determined by the rotor currents IR1-IR3, the power controller312 can control total active and reactive power and the current levelfor each phase on the grid by controlling the rotor currents IR1-IR3.The control of rotor currents IR1-IR3 will be described in more detailregarding the scalar control algorithm detailed in FIG. 5.

Scalar Control Algorithm

FIG. 5 illustrates a flow diagram of a method 500 for performing ascalar control algorithm by the power control 312 of FIG. 4. In oneimplementation, the scalar control algorithm is based on controllingoscillating signals. That is, the scalar control algorithm controlsoscillating rotor currents IR1-IR3 based on, e.g., a sinusoidalwaveform.

Initially, method 500 begins at stage 502, where active power PMG andreactive power QMG are calculated. This stage corresponds withprocessing stage 402 of FIG. 4. The total active power PMG can becalculated instantaneously by using the following equation:p(t)=u ₁(t)·i ₁(t)+u ₂(t)·i ₂(t)+u ₃(t)·i ₃(t)where u_(L1)-u_(L3) correspond to u₁(t)-u₃(t) and i_(L1)-i_(L3)correspond to i₁(t)-i₃(t).

The total reactive power QMG can also be calculated instantaneously byusing the following equation:${q(t)} = {\frac{1}{\sqrt{3}}\left\lbrack {{{i_{1}(t)} \cdot \left( {{u_{3}(t)} - {u_{2}(t)}} \right)} + {{i_{2}(t)} \cdot \left( {{u_{1}(t)} - {u_{2}(t)}} \right)} + {{i_{3}(t)} \cdot \left( {{u_{2}(t)} - {u_{1}(t)}} \right)}} \right\rbrack}$where the instantaneous values for the current i(t) and u(t) can bedescribed as:i(t)=î·sin(ω_(g) t+φ _(i)) and u(t)=û·sin(ω_(g) t+φ _(u))and î is the amplitude of the current, û the amplitude of the voltageand ω_(g) is calculated from the grid frequency f_(g). The powercalculations can be performed for each phase of the grid to obtain rotorcurrent references IR1-IR3 for each phase of the rotor.

At stage 504, a target active power (PMG_(ref)) and a target reactivepower (QMG_(ref)) are derived. The PMG_(ref) value can be calculated inmain controller 310. For example, main controller 310 can use a lookuptable to determine ideal active power for a given measured generatorspeed and rotor current. The QMG_(ref) value can be user selected. Forexample, the QMG_(ref) value can be selected based on either aselectable number of variables or a selected power factor angledepending on the functions and results of reactive power compensationdesired. That is, depending on the different ways that the reactivepower is determined, a final target value QMG_(ref) is derived.

At stage 506, error signals are determined for active power and reactivepower based on calculations using PMG and QMG and PMG_(ref) andQMG_(ref). For example, the PMG_(ref) is compared with PMG to generatean active power error signal and QMG_(ref) is compared with QMG togenerate a reactive power error signal. These error signals could bedetermined for each phase of the grid. This stage corresponds withprocessing stages 403A and 403B of FIG. 4.

Turbine 100 can operate as a doubly-fed turbine with rotor excitationcontrol (as opposed to providing reactive power and power factor controlon the grid or line side). That is, the turbine can provide reactivepower and power factor control on the generator or rotor (or “machine”side) with a control mechanism to regulate the active and reactive powergenerated on the grid by controlling rotor excitation. At stage 508, acurrent reference waveform (IR_(ref)) is determined for the currents inthe three phases of the rotor. This stage calculates current referencewaveforms (IR1_(ref)-IR3_(ref)). The rotor currents can be described asthe sum of current components (active and reactive), where the firstpart is the active component i_(r real) responsible for the active powerand the second component i_(r complex) is the magnetic componentresponsible for the reactive power such that each instantaneous rotorcurrent is:i _(r)(t)=i _(r) _(real) (t)+i _(r) _(complex) (t) and i_(r)(t)={circumflex over (l)} _(r)·sin(ω_(r) t+β)where the angular frequency ω, for the rotor is calculated out of therotor speed ω_(m) and the grid frequency with:ω_(r)=φ_(g) −Ps·ω_(m) Ps:number of pole pairs

The IR1_(ref)-IR3_(ref) values can be calculated in the power controlprocessing stage of FIG. 4 using measured grid frequency and generatorspeed. The calculations can be based on trigonometric functions, wherethe amplitude of the rotor-current Î, is the trigonometric sum of theactive and reactive part of the desired rotor current and the load angleβ (∀β), which is the phase angle between the two components. Forexample, Î, can be calculated using the following equation:Î _(r) =√{square root over (i _(r real) ² +i _(r complex) ² )}and the load angle (∀β) could be calculated using the followingequation:∀β=arctan (i_(r)complex/i_(r) real)

At stage 510, a determination is made if each measured current value orwaveform matches the calculated current reference waveformsIR1_(ref)-IR3_(ref). This stage corresponds to processing stages408A-408C of FIG. 4. If the waveforms match, method 500 continues backto stage 510. If the waveforms do not match, an error is determined andmethod 500 continues to stage 512.

At stage 512, electrical quantities in the rotor are adjusted such thateach measured current waveforms (IR1-IR3) matches the current referencewaveform (IR1_(ref)-IR3_(ref)). This stage corresponds to processingstages 410A-410C, and 420 of FIG. 4. In particular, based on thedetermined error, desired voltage references (UR1 _(ref)-UR3 _(ref)) areset for PWM processing. PWM processing uses these voltage references(UR1 _(ref)-UR3 _(ref)) to control active switches in active inverter151, which control rotor currents IR1-IR3. The above method can becontinuously performed to adjust rotor currents for each phase of therotor thereby controlling electrical quantities for each phase of thegrid.

In a similar manner, the power for each phase of the grid could bedetermined independently. In this case, the rotor currents may becontrolled such that each phase of the grid is controlled independently,making the turbine 100 responsive to asymmetry present on the grid.

Dependent Pitch Control

The main components for providing dependent pitch control are maincontroller 310, power controller 312, power error feed forward 314, andpitch controller 316. The main controller 310 calculates a powerreference and a main pitch reference for the power controller 312 andpitch controller 316, respectively. The internal components of maincontroller 310 to calculate the power reference and main pitch referencewill now be explained.

FIG. 6 illustrates an internal block diagram of one implementation forthe main controller 310 of FIG. 3. Main controller 310 includes a RPMset point calculation 602 and a pitch set point calculation 604providing optimal RPM and pitch set point values. These values arechosen to allow wind turbine 100 to deliver as much electrical energy aspossible. Main controller 310 also includes a partial load controller606, switch logic 607, and full load controller 608.

The RPM set point calculation 602 receives a wind speed measurement toset the RPM set point value. Pitch set point calculation 604 receives ameasured RPM value from the generator and the wind speed measurement toset the pitch set point value. Partial load controller 606 receives themeasured RPM value, a maximum power value, and the RPM set point valueto calculate the power reference (PMG_(ref)). Partial load controller606 ensures the maximum power is not exceeded. FIG. 7 describes infurther detail the manner in which partial load controller 606calculates the power reference (PMG_(ref)). Full load controller 608receives the measured RPM value, pitch set point calculation value, andthe RPM set point calculation value to calculate the main pitchreference. Full load controller 608 ensures that the pitch angle is notlower than the optimal pitch angle. FIG. 8 describes in further detailthe manner in which full load controller calculates the main pitchreference.

Referring to FIG. 6, switch logic 607 provides an enable signal to bothpartial load controller 606 and full load controller 608. The enablesignal controls when portions of the partial load controller 606 andfull load controller 608 are enabled to operate as will be describedbelow in FIGS. 7 and 8.

FIG. 7 illustrates an internal block diagram of one implementation forthe partial load controller 606 of FIG. 6. In one embodiment, partialload controller 606 is active only when the turbine power is operatingat less than maximum power output. Referring to FIG. 7, a comparator 701compares the measured RPM value with RPM set point calculation todetermine an RPM error (e.g., RPM set point—measured RPM). This error issent to PI controller 704 via gain scheduling 702, which also receivesthe RPM set point signal. Gain scheduling 702 allows the amplification(gain) for partial load controller 605 to be dependent on a certainsignal, i.e., the RPM set point signal. PI controller 704 generates thepower reference signal using the error signal from gain scheduling 702.In one embodiment, if the power reference signal exceeds the maximumpower, a signal is sent to switch logic 607 to cause switch logic 607 todisable partial load controller 606 and enable full load controller 608,and the output will be clamped by controller 606 to the maximum power.

FIG. 8 illustrates an internal block diagram of one implementation forthe full load controller 608 of FIG. 6. In one embodiment, full loadcontroller 608 is active only when the wind turbine power is equal tothe maximum power. If the wind speed is high enough, it may produce toomuch power and the turbine components may become overloaded. In thissituation, the RPM generator speed is also controlled by moving thepitch angle away from the maximum power position for the wind blades.

Referring to FIG. 8, a comparator 801 compares the RPM set point withmeasured RPM to determine an RPM error (e.g., RPM set point—measuredRPM). This error is sent to PI controller 805 via gain scheduling I 802and gain scheduling II 804. Gain scheduling 1802 receives the RPM errorand gain scheduling II 804 receives main pitch reference signal. Gainscheduling 1802 and II 804 control gain for full load controller 608dependent on RPM error and main pitch reference. PI controller 805generates the main pitch reference signal using the RPM error. In oneembodiment, if the main pitch reference is lower than the maximum powerset point, a signal is sent to switch logic 607 to cause switch logic607 to disable full load controller 608 and enable partial loadcontroller 606, and the output will be clamped to the maximum powerproducing pitch set point. Main controller 310, however, can use othermore complicated pitch and power reference generating schemes thatensure reduction of loads, noise, etc. For example, partial loadcontroller 606 and full load controller 608 could use the power errorfeed forward signal to quickly react to a large power error.

FIG. 9 illustrates a block diagram of one implementation for the pitchcontroller 316 of FIG. 3. Referring to FIG. 9, pitch controller 316includes a comparator 906 that compares a secondary pitch referencesignal from power error feed forward 314, main pitch reference signalfrom main controller 310, and a measured pitch angle from pitch system910 to determine a pitch error. The pitch error can be, e.g.,[(mainpitch reference+secondary pitch reference)−measured pitch angle]. Anon-linear P-controller 908 provides a control voltage to a pitch system910 based on the pitch error. Pitch system 910 connects with one of thewind blades 301 and includes components to control the pitch of the windblade. For example, pitch system 910 may include a hydraulic systemwhere the control voltage is applied to a proportional value thatgenerates a hydraulic flow moving a pitch cylinder that controls thepitch of a wind blade. The pitch position can be monitored by thedisplacement of the cylinder and feedback to comparator 906. The samplerate for pitch controller 316 can be set at a low value compared to thesample rate for power controller 312. For example, pitch controller 316could operate at 50 Hz while power controller 312 could operate at 5Khz.

Thus, a variable speed wind turbine is provided having a passive gridside rectifier with scalar power control and a pitch controlleroperating dependently with a power controller. Furthermore, while therehas been illustrated and described what are at present considered to beexemplary implementations and methods of the present invention, variouschanges and modifications may be made, and equivalents may besubstituted for elements thereof, without departing from the true scopeof the invention. In particular, modifications may be made to adapt aparticular element, technique, or implementation to the teachings of thepresent invention without departing from the spirit of the invention.

In addition, while the described implementations include hardwareembodiments, which may run software to perform the methods describedherein, the invention may also be implemented in hardware or softwarealone. Accordingly, the software can be embodied in a machine-readablemedium such as, for example, a random access memory (RAM), read-onlymemory (ROM), compact disc (CD) memory, non-volatile flash memory, fixeddisk, and other like memory devices. Furthermore, the processors andcontroller described herein can execute the software to perform themethods described above. Other embodiments of the invention will beapparent from consideration of the specification of the inventiondisclosed herein. Therefore, it is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A method of operating a variable speed wind turbine, comprising:configuring at least two phases of a stator or a generator forconnection to a utility grid; providing a plurality of switches on therotor-side of a generator; and controlling the switches such thatelectrical quantities for at least two phases of the utility grid arecontrolled independently.
 2. A method for controlling power from anelectrical generator to a power grid comprising: receiving voltagevalues and current values of the power grid relative to a time-basedsystem; calculating active and reactive power from the voltage andcurrent values without converting from the time-based system;determining a power error based on the active and reactive power;generating current reference values to control rotor currents in theelectrical generator based on the determined power error withoutconverting from the time-based system; and controlling the rotorcurrents in the electrical generator based on the current referencevalues.
 3. The method of claim 2, wherein the receiving of the voltagevalues and current values includes receiving the voltage and currentvalues for each phase of the power grid.
 4. The method of claim 3,wherein the calculating of the active and reactive power includescalculating the active and reactive power for each phase of the powergrid using the voltage and current values for each of the power grid. 5.The method of claim 2, wherein the generating of the current referencevalues includes generating the current reference values based on thedetermined power error, a power grid frequency, and a generator speed.6. The method of claim 2, wherein the controlling of the rotor currentsin the electrical generator includes controlling the rotor currents foreach phase of the rotor in the electrical generator based on the currentreference values.
 7. The method of claim 2, further comprising:controlling electrical quantities in each phase of the power grid bycontrolling the rotor currents in the electrical generator.
 8. A systemfor controlling power from an electrical generator to a power gridcomprising: means for receiving voltage values and current values of thepower grid relative to a time-based system; means for calculating activeand reactive power from the voltage and current values withoutconverting from the time-based system; means for determining a powererror based on the active and reactive power; means for generatingcurrent reference values to control rotor currents in the electricalgenerator based on the determined power error without converting fromthe time-based system; and means for controlling the rotor currents inthe electrical generator based on the current reference values.
 9. Thesystem of claim 8, wherein the means for generating current referencevalues includes means for generating current reference values withoutconverting from a time-based system.
 10. The system of claim 8, whereinthe means for receiving of the voltage values and current valuesincludes means for receiving the voltage and current values for eachphase of the power grid.
 11. The system of claim 10, wherein the meansfor calculating of the active and reactive power includes means forcalculating the active and reactive power for each phase of the powergrid using the voltage and current values for each of the power grid.12. The system of claim 8, wherein the means for generating of thecurrent reference values includes means for generating the currentreference values based on the determined power error, a power gridfrequency, and a generator speed.
 13. The system of claim 8, wherein themeans for controlling of the rotor currents in the electrical generatorincludes means for controlling the rotor currents for each phase of therotor in the electrical generator based on the current reference values.14. The system of claim 8, further comprising: means for controllingelectrical quantities in each phase of the power grid by controlling therotor currents in the electrical generator.
 15. A method for a variablespeed wind turbine comprising: generating rotor currents for anelectrical generator connected to wind blades, the rotor currentscorresponding to a plurality of phases of a power grid; and controllingindependently the rotor currents for each phase of the power grid. 16.The method of claim 15, wherein controlling independently the rotorcurrents includes controlling independently rotor currents for eachphase of the power grid based on electrical characteristics of eachphase of the power grid.
 17. A variable speed wind turbine comprising: agenerator connected to wind blades that provides rotor currents, therotor currents corresponding to a plurality of phases of a power grid;and a controller to independently control the rotor currents for eachphase of the power grid.
 18. The variable speed wind turbine of claim17, wherein the controller controls independently rotor currents foreach phase of the power grid based on electrical characteristics of eachphase of the power grid.
 19. A method for a variable speed wind turbinecomprising: generating electrical power for a power grid using a rotorand stator of an electrical generator; and controlling active switchesin a power converter using a scalar control algorithm that controls eachphase of the rotor based on measured electrical quantities of each phaseof the power grid.
 20. The method of claim 19, wherein controlling theactive switches includes controlling the active switches using pulsewidth modulation (PWM) techniques.
 21. The method of claim 19, furthercomprising: receiving at least power grid frequency values and rotorcurrent values for each phase of the rotor to control the activeswitches.
 22. The method of claim 19, further comprising: controllingthe electrical power to the power grid without performing a coordinatetransformation of AC signals.
 23. A variable speed wind turbinecomprising: an electrical generator having a rotor and stator togenerate electrical power for a power grid; and a power converter tocontrol the generated electrical power, the power converter includingactive switches controlled by a scalar control algorithm that controlseach phase of the rotor based on measured electrical quantities of eachphase of the power grid.
 24. The variable speed wind turbine of claim23, further comprising a controller to control the active switches ofthe power converter.
 25. The variable speed wind turbine of claim 24,wherein the controller controls the active switches using pulse widthmodulation (PWM) techniques.
 26. The variable speed wind turbine ofclaim 24, wherein the controller receives at least power grid frequencyvalues and rotor current values for each phase of the rotor to controlthe active switches.
 27. The variable speed wind turbine of claim 26,wherein the controller controls electrical power to the power gridwithout performing a coordinate transformation of AC signals.
 28. Amethod for a power controller of a variable speed wind turbinecomprising: receiving power grid related information for a power grid;calculating active and reactive power values for the power grid based onthe received power grid related information; comparing the calculatedactive and reactive power values with reference active and reactivepower values, respectively; generating current reference values based onthe comparison of the calculated active and reactive power values withthe reference active and reactive power values; comparing the generatedcurrent reference values with measured current values; and generatingnew current values to control the active and reactive power for thepower grid based on the comparison of the generated current referencevalues with the measured current values.
 29. The method of claim 28,wherein the power grid related information includes voltage and currentmeasurement values for each phase of the power grid.
 30. The method ofclaim 28, further comprising: generating control signals to controlactive switches that output the new current values.
 31. The method ofclaim 30, wherein the active switches are controlled using pulse widthmodulation (PWM) techniques.
 32. A computer-readable medium containinginstructions, which if executed by a computing system, causes thecomputing system to perform a method comprising: receiving voltagevalues and current values of a power grid relative to a time-basedsystem; calculating active and reactive power from the voltage andcurrent values without converting from the time-based system;determining a power error based on the active and reactive power;generating current reference values to control rotor currents in anelectrical generator based on the determined power error withoutconverting from the time-based system; and controlling the rotorcurrents in the electrical generator based on the current referencevalues.
 33. A computer-readable medium containing instructions, which ifexecuted by a computing system, causes the computing system to perform amethod comprising: receiving power grid related information for thepower grid; calculating active and reactive power values for the powergrid based on the received power grid related information; comparing thecalculated active and reactive power values with reference active andreactive power values, respectively; generating current reference valuesbased on the comparison of the calculated active and reactive powervalues with the reference active and reactive power values; comparingthe generated current reference values with measured current values; andgenerating new current values to control the active and reactive powerfor the power grid based on the comparison of the generated currentreference values with the measured current values.